A linear image sensor consisting of a one-dimensional array of photodiodes is sometimes used in a bar-code reader system or the like and in this case the photodiodes need to have an elongated shape in a direction perpendicular to their array direction. Patent Literature 1 discloses the linear image sensor of this type.
In the linear image sensor described in Patent Literature 1, a plurality of light receiving portions with pn junction type photodiodes of an elongated shape are arranged in a one-dimensional array. In this linear image sensor, each pn junction type photodiode is formed of an n-type semiconductor substrate and a p-type semiconductor region formed on the n-type semiconductor substrate, and an amount of charge according to an intensity of incident light is accumulated in a pn junction capacitance formed by these n-type semiconductor substrate and p-type semiconductor region. In this linear image sensor, a transistor is formed adjacently to the pn junction type photodiode and the charge accumulated in the pn junction type photodiode is read out by this transistor. In this linear image sensor, however, since the charge is accumulated in the pn junction capacitance of the pn junction type photodiode, if the pn junction region becomes large in the elongated shape, its pn junction capacitance also becomes large, causing a problem of reduction in response speed.
With respect to this problem, there is a conventionally-developed linear image sensor using embedded photodiodes instead of the pn junction type photodiodes. In each embedded photodiode, for example, an n-type low-concentration semiconductor region is formed on a p-type substrate and a thin p-type high-concentration semiconductor region is formed on a surface of this n-type low-concentration semiconductor region. In this embedded photodiode, since the n-type low-concentration semiconductor region can be made completely depleted, an apparent pn junction capacitance in readout of charge can be made zero. As a result, the response speed can increase.